Abstract: Objective To study the impact of co-infection of hepatitis C virus and Mycobacterium tuberculosis on patient's natural killer (NK) cells and explore the immunological function of NK in the patients co-infected with hepatitis C virus and M. tuberculosis. Methods Totally 47 patients with simple hepatitis C infection, 66 patients with simple infection of M. tuberculosis and 27 patients co-infected with hepatitis C virus and M. tuberculosis were selected. The NK cell counts and secreted cytokines contains in theirs peripheral blood were detected with flow cytometry (FCM). Results The NK cell counts and IFN-γ contains in peripheral blood of the patients co-infected with hepatitis C virus and M. tuberculosis were obviously lower than those of the patients with simple hepatitis C infection or simple M. tuberculosis infection,the differences were statistical significant (PConclusion The co-infection reduced the NK cell counts and activity in peripheral blood. Cellular immunity was involved in the development and progress of co-infection of hepatitis C virus and M. tuberculosis, which should be considered in clinical diagnosis and treatment.